Regeneration system for an exhaust gas cleaning device

ABSTRACT

A regeneration system for an exhaust gas cleaning device disposed in an exhaust emission path of an internal combustion engine comprises an exhaust gas cleaning honeycomb filter and a heating means for the filter, wherein the filter is a checkered SiC honeycomb filter having a given cell structure, and the heating means is a heater or a glow plug when using a fuel containing fuel additive.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a regeneration system for an exhaustgas cleaning device, and more particularly to a regeneration systemcomprising an exhaust gas cleaning device disposed in an exhaustemission path of an internal combustion engine and provided with anexhaust gas cleaning honeycomb filter for catching particulates includedin the exhaust gas and a heating means for the exhaust gas cleaninghoneycomb filter when a fuel containing a fuel additive for mitigatingparticulates included in the exhaust gas is used as a fuel for theinternal combustion engine.

[0003] 2. Description of Related Art

[0004] In the internal combustion engine such as diesel engine or thelike, particulates (e.g. soot or unburned portion of fuel) are includedin the exhaust gas. Particularly, the discharge amount of particulatesbecomes large in a diesel engine using a gas oil as a fuel or adirect-injection type gasoline engine recently coming into wide use.Therefore, it is well-known to remove the particulates by an exhaust gascleaning device disposed in an exhaust emission path of the internalcombustion engine and provided with an exhaust gas cleaning honeycombfilter.

[0005] As the exhaust gas cleaning honeycomb filter is generally used acordierite filter 32 of a honeycomb structure as shown by adiagrammatically section view in FIG. 1. In such a conventionalcordierite filter 32 are included a plurality of exhaust gas flowingchannels 33 extending in parallel to a longitudinal direction thereof,wherein these channels 33 are alternately plugged at either upstreamside or downstream side for the exhaust gas of their ends with pluggingmembers 33 a to form a checker pattern.

[0006] As shown in FIG. 1, an exhaust gas Gin emitted from a dieselengine (not shown) flows into the cordierite filter 32 through theexhaust emission path 11, at where particulates included in the exhaustgas are filtered off on surfaces of cell walls constituting the exhaustgas flowing channels 33. Then, the cleaned exhaust gas Gout passedthrough cordierite filter 32 again passes through the exhaust emissionpath 11 and is discharged out to the outside of the vehicle.

[0007] It is known that pressure loss ΔP is produced when the exhaustgas Gin passes through the filter 32. The pressure loss ΔP isrepresented by the following equation (1).

ΔP=ΔP1+ΔP2+ΔP3+ΔP4  (1)

[0008] wherein

[0009] ΔP1 is a resistance produced due to the narrowing of an openingportion in the exhaust gas flowing channel 33 when the exhaust gas flowsinto the channel 33 through the exhaust emission path 11;

[0010] ΔP2 is a resistance produced in the flowing of the exhaust gasthrough the exhaust gas flowing channel 33;

[0011] ΔP3 is a resistance produced in the passing through a wall of theexhaust gas flowing channel 33;

[0012] ΔP4 is a resistance produced when the exhaust gas passes throughparticulates deposited on the surface of the exhaust gas flowing channel33.

[0013] In this case, the resistances ΔP1, ΔP2, ΔP3 are dependent upon acell structure constituting the filter 32, respectively and are aconstant value ΔPi not depended upon the lapse of time coming intoproblem in the deposition of the particulates and the like (hereinafter“ΔP1+ΔP2+ΔP3” is called as “initial pressure loss”). For this end, agreater part of the pressure loss ΔP is determined by the resistance ΔP4produced when the exhaust gas passes through the particulates depositedon the cell walls. The resistance ΔP4 is usually 2-3 times the initialpressure loss ΔPi at a deposited state of the particulates.

[0014] In FIG. 2 is shown a relation among cell structure, typicaldimensions, geometrical surface area and opening ratio in the filter.The cell structure Cs (mil/cpi) is represented by thickness of cell walldc (mil=milli inch) to cell number Nc per square inch (cpi=cells persquare inch), and the geometrical surface area fs (cm²/cm³) is an areapassing the exhaust gas per unit volume (filtering area). Moreover, thecell wall thickness dc is shown by unit of mm in FIG. 2.

[0015] As seen from FIG. 2, the pressure loss ΔP produced in thecheckered honeycomb filter for cleaning the exhaust gas is small as thecell number Nc and geometrical surface area fs in the filter becomelarge. And also, the opening ratio α (%) is a ratio of total openingarea of the exhaust gas flowing channels occupied in the sectional areaof the filter. As shown in FIG. 2, a limit not creating cracks (cracklimit) is large as the opening ratio α becomes small.

[0016] On the other hand, a mechanical strength of the filter, i.e.bending strength S* of the filter is approximately equal to product ofstrength S of a filter made of porous material and relative density ρ*as mentioned below. When the nature of the porous material constitutingthe filter is represented by density ρ and strength S, the bendingstrength S* of the filter and the relative density ρ* are as follows:

ρ*=α×ρ  (2)

S*≅ρ*×S  (3)

[0017] That is, the strength is high as the opening ratio a becomessmall.

[0018] Further, the regeneration of the filter is carried out by burningthe particulates according to the following reaction equation (4):

C+O₂→CO₂+Q(heat quantity)  (4),

[0019] so that the strength of the filter against the thermal stressbecomes important. Particularly, when the filter is made from a ceramicmaterial, brittle rupture is caused by thermal stress to create cracks.Such a cracking phenomenon is apt to be created as heat quantityproduced in the regeneration or the amount of the particulates depositedto be burnt becomes large. Moreover, the unburned portion of fuelconstituting the particulate is an organic compound, so that it isburned by heating the filter. As mentioned below, a crack limitpreventing the occurrence of the cracking phenomenon is proportional tothe opening ratio α and is closely related to the thickness dc of thecell wall as seen from FIG. 2. If the opening ratio α is same, as thethickness dc of the cell wall becomes thick, the crack limit is high.

[0020] Therefore, the exhaust gas cleaning filter having good propertiesis preferably made of a material having a large crack limit, anexcellent strength against thermal stress and a small pressure loss.

[0021] Recently, fuel previously including a fuel additive, or a devicedropwise adding a fuel additive to a fuel is developed for controllingthe amount of the particulate produced in the exhaust gas and the usethereof is increasing. Such a fuel additive has an effect of preventingthe formation of soot in the burning of the fuel.

[0022] However, the formation of the particulate can not completely becontrolled even by using such a fuel additive and hence the particulateis formed in the exhaust gas. Therefore, it is indispensable to use theexhaust gas cleaning filter.

[0023] In the conventional technique, the cordierite filter is generallyadopted as a checkered honeycomb filter for cleaning the exhaust gas aspreviously mentioned. However, there is a problem that the amount of theparticulate to be treated in one regeneration of the cordierite filterhas a limit because the maximum service temperature in the filter islow. In this case, a large pressure loss is caused in the filter due tothe deposition of the particulate, so that the combustion efficiency ofthe internal combustion engine lowers to degrade the fuel consumption.

[0024] And also, there is proposed a technique for regenerating theexhaust gas cleaning device by burning the particulate caught on thecordierite filter through a heating means for the filter. However, whensuch a greater amount of the particulate caught on the filter is burntout by the heating means at once, a large change of the pressure loss iscaused in the burning of the particulate in accordance with the heatconduction efficiency of the heating means, which gives incompatiblefeeling to a driver.

[0025] In FIG. 3 are shown experimental data illustrating a relationamong pressure loss ΔP (mmAq) and temperature T (° C.) and time t (min)in the conventional checkered honeycomb cordierite filter for cleaningthe exhaust gas. In FIG. 3, symbol Po is a case of burning a fuelcontaining a fuel additive, wherein as the temperature T rises with theincrease of engine revolution number (engine loading), the deposition ofparticulates begins to decrease on the border of a certain time. Thatis, the burning of the particulate is begun at a temperature of To=about380° C. to conduct the regeneration of the filter.

[0026] On the other hand, symbol Pn is a case of burning a fuelcontaining no fuel additive, wherein the pressure loss ΔP in the filtercontinuously rises in proportion to the deposition of the particulateeven when the temperature Tn rises with the increase of the enginerevolution number (engine loading). As a result, the burning of theparticulate at a temperature of Tn=about 380° C. is not carried outdifferent from the case of burning the fuel containing the fueladditive. Moreover, the beginning temperature of burning theparticulates in case of using the fuel containing no fuel additive isgenerally about 630° C.

[0027] In order to reduce the pressure loss produced in the cordieritefilter, therefore, it is considered to finely set the cell structure Csof the cordierite filter with reference to FIG. 2. For example, it isconsidered that the cell number Nc (cpi) is set to a large value, whilethe thickness dc of the cell wall is set to a small value.

[0028] In the conventional cordierite filter, however, there is a limitin the formation of the fine cell structure from a view point of thestrength inherent to the cordierite. For example, it is possible tomanufacture the cordierite honeycomb filter having the cell number Nc ofmore than 100 cpi, but when such a filter is used as a checkeredhoneycomb filter capable of efficiently burning the particulate, thecracking is caused in view of the crack limit of cordierite itself, sothat the cell number Nc (cpi) can not be made more than 100 when thecheckered honeycomb filter for cleaning the exhaust gas is made ofcordierite.

[0029] In addition, if it is intended to burn the deposited particulatesthrough only the temperature of the exhaust gas, there is caused aninconvenience that the honeycomb filter for cleaning the exhaust gas cannot completely be regenerated because the exhaust gas may not rise to atemperature required for burning the particulate when the vehicle isfrequently run on urban area at a low speed.

SUMMARY OF THE INVENTION

[0030] Under the above situations, it is an object of the invention toprovide a regeneration system for an exhaust gas cleaning devicedisposed in an exhaust emission path of an internal combustion enginecapable of controlling a change of pressure loss to a smaller valueduring the regeneration and conducting the complete regeneration of thefilter even at a running state hardly raising the temperature of theexhaust gas by using an exhaust gas cleaning honeycomb filter made of aporous silicon carbide sintered body, which can be set to a fine cellstructure and is high in the crack limit and excellent in the strengthagainst thermal stress, together with a heating means for the filterwhen a fuel containing a fuel additive is used as a fuel for theinternal combustion engine.

[0031] According to the invention, there is the provision of aregeneration system for an exhaust gas cleaning device disposed in anexhaust emission path of an internal combustion engine comprising anexhaust gas cleaning honeycomb filter for collecting particulatesincluded in the exhaust gas and a heating means for the exhaust gascleaning honeycomb filter, characterized in that said internalcombustion engine uses a fuel containing a fuel additive, and saidfilter is a checkered honeycomb filter made of a porous silicon carbidesintered body and having a cell structure that a cell number per squareinch is not less than 100 cells and a thickness of a cell wall is notmore than 0.43 mm, and said heating means is selected from a heater anda glow plug.

[0032] In a preferable embodiment of the invention, the glow plug is aceramic glow plug.

[0033] In another preferable embodiment of the invention, the filter hasa total volume corresponding to ¼-2 times an engine swept volume of theinternal combustion engine.

[0034] In the invention, the exhaust gas cleaning honeycomb filter is acheckered SiC honeycomb filter made of the porous silicon carbidesintered body, so that it is high in the crack limit and excellent inthe strength against thermal stress as compared with those of theconventional cordierite filter and hence the durability of the filter ishigh. And also, the checkered SiC honeycomb filter has a cell structurethat the cell number per square inch is not less than 100 cells and thethickness of the cell wall is not more than 0.43 mm (=17 mil), which canbe made finer than that of the conventional cordierite filter, so thatthe pressure loss can be decreased as compared with that of thecordierite filter to improve the fuel consumption.

[0035] Furthermore, in the regeneration system according to theinvention, the heater or the glow plug as the heating means for theexhaust gas cleaning filter is arranged ahead the filter in the exhaustgas cleaning device, so that even if the temperature of the exhaust gasis not raised to a level required for burning the particulate during therunning of the vehicle on urban area at a low speed or the like, theparticulate can be burned by heating the filter through such a heatingmeans, and hence the regeneration of the filter can completely beattained.

[0036] In case of using the glow plug, the space in the exhaust gascleaning device can be saved and power consumption can be decreased ascompared with those in the use of the heater. As the glow plug, thereare a metal glow plug and a ceramic glow plug. The use of the ceramicglow plug is favorable because the time reaching to a given temperatureby heating is faster than that of the metal glow plug and a saturatedtemperature can be made higher. And also, the ceramic glow plug is smallin the power consumption as compared with the metal glow plug and ishigh in the durability. Therefore, the more efficient regeneration ofthe filter can be attained by the use of the ceramic glow plug ascompared with the use of the heater or the metal glow plug.

[0037] In the invention, the total volume of the exhaust gas cleaningfilter is set to ¼-2 times the engine swept volume of the internalcombustion engine because the total volume is dependent upon the engineswept volume. When the total volume of the filter is less than ¼ timesthe engine swept volume, the sufficient filtering area can not beensured and the pressure loss becomes larger to bring about theremarkable degradation of the fuel consumption, while when it exceeds2.0 times, it is difficult to arrange the exhaust gas cleaning deviceinclusive of the filter in the exhaust emission path but also thethermal capacity becomes larger to delay reaction to the temperature ofthe exhaust gas to thereby lose a chance of obtaining a regeneratabletemperature of the filter for burning the particulate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The invention will be described with reference to theaccompanying drawings, wherein:

[0039]FIG. 1 is a diagrammatically section view illustrating theconventional checkered cordierite honeycomb filter for cleaning exhaustgas;

[0040]FIG. 2 is a graph showing a relation among cell structure, typicaldimensions, geometrical surface area and opening ratio in the filter;

[0041]FIG. 3 is a graph showing a pressure loss produced when acheckered honeycomb filter for cleaning exhaust gas is made ofcordierite in consideration with the presence or absence of a fueladditive as a function of temperature and time in the filter;

[0042]FIG. 4 is a block schematic diagram of a first embodiment of theregeneration system for the exhaust gas cleaning device according to theinvention;

[0043]FIG. 5 is a diagrammatically section view of a main part in theregeneration system according to the invention shown in FIG. 4;

[0044]FIG. 6a is a perspective view of a checkered honeycomb SiC filterfor cleaning an exhaust gas according to the invention;

[0045]FIG. 6b is a partial front view of the checkered honeycomb SiCfilter shown in FIG. 6a;

[0046]FIG. 7a is a diagrammatic view partly shown in section of aceramic glow plug used as a heating means;

[0047]FIG. 7b is a diagrammatic view partly shown in section of a metalglow plug used as a heating means;

[0048]FIG. 8 is a view showing a comparison in properties between theceramic glow plug and the metal glow plug;

[0049]FIG. 9 is a chart illustrating a time period until a givenpressure loss is caused by the presence or absence of an exhaust gaspurification catalyst for an exhaust gas cleaning device;

[0050]FIG. 10 is a graph showing a comparison in pressure loss between acheckered SiC honeycomb filter and a checkered cordierite honeycombfilter when using a gas oil containing a fuel additive;

[0051]FIG. 11 is a graph showing a pore size distribution of a cell wallin the conventional cordierite filter and the SiC filter according tothe invention;

[0052]FIG. 12a is a diagrammatic view of an inner structure of a cellwall in the SiC filter according to the invention;

[0053]FIG. 12b is a diagrammatic view of an inner structure of a cellwall in the conventional cordierite filter;

[0054]FIG. 13 is a graph showing a relation between initial pressureloss and air flow rate in the checkered honeycomb SiC filter;

[0055]FIG. 14 is a graph showing an influence of a diameter φ of thecheckered SiC honeycomb filter upon pressure loss ΔP when using a gasoil containing a fuel additive;

[0056]FIG. 15 is a graph showing a relation between oxygen concentrationin exhaust gas and temperature burning particulate for attaining aregeneration ratio of not less than 80% in the exhaust gas cleaningfilter when the invention is compared with the conventional technique;and

[0057]FIGS. 16a and 16 b are diagrammatic views illustrating anotherembodiments of the regeneration system according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0058] In FIG. 4 is shown a first embodiment of the regeneration systemfor the exhaust gas cleaning device for use in a diesel engine accordingto the invention. This system is a system for the regeneration of anexhaust gas cleaning device 20 cleaning the exhaust gas discharged froma diesel engine 10 using a gas oil containing a fuel additive, andcomprises three regeneration units 12-1, 12-2, 12-3 disposed on a way ofan exhaust emission path 11 from the engine 10 in a branched form. Eachof these units comprises an exhaust gas cleaning device 20 provided withan exhaust gas cleaning filter 22 made of a porous silicon carbide (SiC)sintered body and carried with a catalyst for exhaust gas purification,a heating means 24 disposed ahead an end face of the SiC filter 22 at anupstream side thereof for heating the SiC filter 22, and a flow controlvalve 26 disposed at a downstream side of the SiC filter 22. Further,the regeneration system comprises a control unit 40 for controllingpower supplied to the heating means 24 and the flow control valve 26.The control unit 40 is connected to a battery 28 mounted on a vehicle(not shown) and controls the heating means 24 and the flow control valve26 independently.

[0059] The control unit 40 comprises an input part 42 inputted withmeasured values of engine revolution number Ne (rpm) and pressure lossΔP (mmAq) and temperature T in an exhaust gas cleaning device 20, CPU(central processing unit) 44 processing output signals from the inputpart 42 for controlling actions of the heating means 24 and the flowcontrol valve 26, a heater controlling unit 46 operating the heatingmeans 24 based on the processed result in CPU 44, and an output part 48operating the flow control valve 26 to a throttle position (see aposition of a valve body 26 a in the regeneration unit 12-1 of FIG. 4).

[0060] As shown in FIG. 5, the heating means 24 is a spiral-shapedresistance heating heater disposed apart from the upstream end face ofthe SiC filter 22 at a given distance. The heater is not restricted tothe above spiral form and may take any forms as long as the exhaust gasfrom the engine 10 is passed in the arrangement ahead the SiC filter 22.Moreover, the kind of the heater is not particularly restricted, but asheathed heater and the like may be used.

[0061] The honeycomb filter 22 for cleaning the exhaust gas will bedescribed in detail below. In FIG. 5 is diagrammatically shown a firstregeneration unit 12-1 among the three regeneration units 12-1, 12-2 and12-3 in the exhaust gas cleaning device. As shown in FIG. 5, thehoneycomb filter 22 for cleaning the exhaust gas is made of a poroussilicon carbide (SiC) sintered body of a honeycomb structure. In such aSiC honeycomb filter 22 are formed a plurality of exhaust gas flowingchannels 23 extending in parallel to a longitudinal direction thereof,wherein these channels 23 are alternately plugged at either inlet sideor outlet side for the exhaust gas of their ends with plugging members23 a to form a checker pattern. That is, the exhaust gas cleaninghoneycomb filter 22 made of the porous silicon carbide sintered body isa checkered SiC honeycomb filter having a given cell wall thickness dcat the upstream and downstream end faces thereof (see FIG. 6).

[0062] When using a fuel containing a fuel additive, harmful componentsincluded in the exhaust gas is decreased but are not completely removedas previously mentioned. Therefore, it is favorable to carry a catalystfor the purification of the exhaust gas on an inner wall surface of theexhaust gas flowing channel 23 for removing these harmful components. Asthe catalyst for the purification of the exhaust gas, use may be made ofthe conventionally known catalysts. Moreover, the checkered SiChoneycomb filter 22 is closely held in a casing of the exhaust gascleaning device 20 through a heat insulating material 23 b arranged onthe outer periphery of the filter.

[0063] The checkered SiC honeycomb filter according to the invention ismade of the porous silicon carbide sintered body so as to have a cellstructure that the cell number Nc per square inch is not less than 100cells and a cell wall thickness dc is not more than 0.43 mm (=17 mil).As a numerical value of a more preferable cell structure Cs (mil/cpi),there are concretely mentioned Cs=14/200, 12/200, 12/300 and the like.

[0064] A second embodiment of the regeneration system for the exhaustgas cleaning device for use in a diesel engine according to theinvention has the same structure as in the first embodiment except thata glow plug 34 is used instead of the heater as the heating means 24 asshown in FIG. 7. As the glow plug 34, there are two plugs as shown inFIGS. 7a and 7 b. The plug shown in FIG. 7a is a ceramic glow plug 34 ausing an electrically conductive ceramic 35 attached to a top of theplug as a heating portion, wherein heat is generated by flowing currentthrough the electrically conductive ceramic 35. The plug shown in FIG.7b is a metal glow plug 34 b using a heating coil 36 attached to the topof the plug as a heating part, wherein heat is generated by flowingcurrent through the heating coil 36.

[0065] In FIG. 8 is shown a comparison in properties between the ceramicglow plug 34 a and the metal glow plug 34 b. In this case, the timereaching to 800° C. by heating is a surface temperature rising rate ofthe heating portion at the top of the plug, and the saturatedtemperature is a temperature when the surface temperature of the glowplug arrives at a saturated state or a temperature capable of conductingthe burning at a stable state.

[0066] As seen from FIG. 8, the time reaching to 800° C. by heating is3.5 seconds in the ceramic glow plug 34 a and 5.5 seconds in the metalglow plug 34 b. On the other hand, the saturated temperature is 1100° C.in the ceramic glow plug 34 a, while that in the metal glow plug 34 b isas low as 900° C. And also, the power consumption per one plug is 36 Win the ceramic glow plug 34 a, while that in the metal glow plug 34 b isrequired to be 55 W. Further, service life of the ceramic glow plug 34 aconverted to a running distance is not less than 200,000 km, while thatof the metal glow plug 34 b is about 100,000 km.

[0067] Therefore, when the ceramic glow plug 34 a is compared with themetal glow plug 34 b, it is effective to use the ceramic glow plug 34 aas a glow plug used as a heating means for the filter.

[0068] The action of the regeneration system according to the inventionwill be described with reference to FIGS. 4 and 5 below. Although theaction is described by using the ceramic glow plug 34 b as the heatingmeans 24, the similar action and effect are obtained even when theheater or the metal glow plug is used as the heating means 24.

[0069] When the exhaust gas Gin emitted from the diesel engine using agas oil containing a fuel additive is flowed into the checkered SiChoneycomb filter 22 at a state of mitigating the amount of particulatein the exhaust gas owing to the use of the fuel additive, theparticulate included in the exhaust gas is filtered off on the surfacesof the exhaust gas flowing channels 23. Thus, the exhaust gas Goutcleaned through the checkered honeycomb SiC filter 22 is again flowedinto the exhaust emission path 11 and discharged out therefrom to theoutside of the vehicle.

[0070] When the diesel engine is worked over a long time, theparticulate included in the exhaust gas is gradually deposited on theinner wall surfaces of the channels 23. Therefore, it is required toregenerate the exhaust gas cleaning device by heating the honeycomb SiCfilter every a given time during the working of the diesel engine.

[0071] During the running of the vehicle, the flow control valve 26 ineach of the three regeneration units 12-1, 12-2, 12-3 is at a non-workedstate, so that when the exhaust gas Gin is passed through the exhaustgas cleaning device 22, the particulate included in the exhaust gas Ginis filtered off by the SiC honeycomb filter 22 and gradually depositedon the inner wall surfaces of the channels 23 in the filter 22.

[0072] When a given amount of the particulate is caught on the innerwall surface of the channel 23, it is difficult to pass the exhaust gasGin through the channel 23, so that the temperature inside the exhaustgas cleaning device 22 rapidly rises. When the temperature inside thecleaning device reaches to a given value with such a temperature rise,the particulate is burnt out by reacting with oxygen included in theexhaust gas Gin. In the running on urban area and the like frequentlyusing low-speed running, however, there is a case that the temperatureof the exhaust gas is not raised to a level required for burning theparticulate.

[0073] In the regeneration system according to the invention, when theparticulate is caught on the surface of the channel over a certain time,for example, only the regeneration unit 12-1 among the threeregeneration units 12-1, 12-2, 12-3 is worked, while the otherregeneration units 12-2, 12-3 are maintained at the non-worked state.That is, the ceramic glow plug 34 a in the regeneration unit 12-1 isswitched-on through a circuit R1 by a command from the control unit 40to heat the end face of the SiC honeycomb filter 22 at the upstream sidethereof, while the valve body 26 a of the flow control valve 26 is alsoworked by a signal V from the output part 48 to throttle a flow of theexhaust gas passing through the SiC honeycomb filter 22 at a downstreamside of the filter. In the regeneration unit 12-1, therefore, theexhaust gas Gin heated by the ceramic glow plug 34 a gently passesthrough the SiC honeycomb filter 22, during which the particulatescaught in the SiC honeycomb filter 22 are burnt out by reacting withoxygen included in the exhaust gas.

[0074] While the SiC honeycomb filter 22 in the regeneration unit 12-1is regenerated by controlling only the ceramic glow plug 34 a and theflow control valve 26 through the control unit 40, the exhaust gas iscleaned by the other remaining regeneration units 12-2 and 12-3.

[0075] After the completion of the regeneration in the regeneration unit12-1, the other remaining regeneration units 12-2 and 12-3 aresuccessively regenerated in the same manner as described above throughthe command from the control unit 40. Therefore, the regeneration units12-1, 12-2, 12-3 are separately and successively regenerated during therunning of the vehicle, which is fairly economical as compared with thecase of regenerating these regeneration units after the running of thevehicle is stopped.

[0076] The effect by the provision of an oxidation catalyst on the innerwall surface of the exhaust gas flowing channel 23 in the SiC honeycombfilter 22 will be described in detail below.

[0077] In FIG. 9 is a chart illustrating a time period until a givenpressure loss is caused by the presence or absence of the oxidationcatalyst as a catalyst for the purification of the exhaust gas in theexhaust gas cleaning device.

[0078]FIG. 9a shows a case that a diesel bus provided with an exhaustgas cleaning device using no oxidation catalyst is run on urban area ata usual low-speed, wherein a regeneration period of the SiC honeycombfilter 22 at a pressure loss of 3000 mmAq produced when 40 g ofparticulates are caught as a measure of beginning the regeneration is 2hours. On the contrary, FIGS. 9b and 9 c show a case of using an exhaustgas cleaning device with an oxidation catalyst, respectively, whereinthe regeneration period based on the pressure loss of 3000 mmAq is 4hours in the low-speed running on urban area or the like as shown inFIG. 9b and 8 hours in the high-speed running on suburbs and expresswaysas shown in FIG. 9c.

[0079] As seen from these charts, when the oxidation catalyst is carriedon the inner wall surface of the exhaust gas flowing channel 23 in theSiC honeycomb filter 22, the operation period for the regenerationsystem can be delayed during the running of the vehicle, which iseffective to improve the durabilities of the ceramic glow plug 34 a andthe flow control valve 26. Moreover, the running mode of the vehicle isusually a mixed mode of low-speed running and high-speed running, sothat the operation of the regeneration system is desirable to be begunevery 3-4 hours as seen from FIGS. 9b and 9 c.

[0080] In FIG. 10 is shown a comparison between the conventionalcheckered honeycomb cordierite filter and the checkered SiC honeycombfilter according to the invention illustrating a change of pressure lossΔP with the lapse of time when a gas oil containing a fuel additive isused while gradually raising a temperature T of the exhaust gas. In thiscase, a curve Pc shows a change of pressure loss in the conventionalcheckered cordierite honeycomb filter having a cell structure Cs(mil/cpi) of 17/100, and curves P1, P2 and P3 show a change of pressureloss in the checkered SiC honeycomb filter according to the inventionhaving cell structures Cs (mil/cpi) of 17/100, 14/200 and 12/300,respectively.

[0081] At first, the conventional cordierite filter is compared with theSiC filter at the same cell structure. In the conventional cordieritefilter, when the particulate included in the exhaust gas is caught to agiven amount by the filter, the change of pressure loss by thecollection of particulates included in the exhaust gas Gin becomesequilibrium at an exhaust gas temperature Tc of about 380° C. or at apressure loss ΔP of not less than 1750 mmAq as shown by the curve Pc inFIG. 10. After the lapse of a given time at the equilibrium state, theparticulate is burnt by raising the exhaust gas temperature Tc to agiven level, whereby the pressure loss is largely reduced. Such a largechange of pressure loss Pc brings about the bad feeling in the runningof the vehicle. Particularly, when the exhaust gas temperature is raisedby pressing down an accelerator toward a floor, rapid burning of theparticulate and rapid reduction of pressure loss are simultaneouslycaused to increase the engine revolution number to an unexpected levelof a driver.

[0082] In the checkered SiC honeycomb filter according to the inventionhaving the same cell structure as in the cordierite filter, when theparticulate is caught by the filter to the same amount as in thecordierite filter, the change of pressure loss becomes equilibrium at anexhaust gas temperature T1 of about 380° C. or at a pressure loss ΔP of1250 mmAq as shown by the curve P1 in FIG. 10. Even when the particulateis burnt by raising the exhaust gas temperature T1 to a given level inthe same manner as in the cordierite filter, the change of pressure lossis small, which brings about comfortable feeling in the running of thevehicle.

[0083] Then, when the cell structure of the SiC filter is changed from17/100 to 14/200 or 12/300, the change of pressure loss when the sameamount of the particulate is caught becomes equilibrium at a pressureloss ΔP of about 950 mmAq or about 700 mmAq, respectively, at theexhaust gas temperature T of about 380° C. That is, as the thickness ofthe cell wall becomes thin and the cell number becomes large, the valueof the equilibrium pressure loss becomes low and hence the change ofpressure loss becomes small.

[0084] As seen from FIG. 10, the amount of the particulate caught whenthe change of pressure loss is at the equilibrium state is the same inany one of the above filters and the value of the equilibrium pressureloss becomes low as the cell structure of the filter becomes fine. Andalso, when the particulate is burnt by raising the exhaust gastemperature, the pressure loss returns to substantially the same levelin the same treating time in all of these filters. Furthermore, as thelevel of pressure loss becomes lower, the change of pressure loss issmall. From these facts, it can be seen that it is favorable to use afilter having a lower level of pressure loss as a honeycomb filter forcleaning the exhaust gas for improving the fuel consumption.

[0085] In FIG. 11 is shown a pore size distribution in a cell wall inthe conventional cordierite filter and the SiC honeycomb filteraccording to the invention, wherein a solid line A indicates the cellwall of the SiC filter and a dot-dash line B indicates the cell wall ofthe cordierite filter. As seen from FIG. 11, the cordierite filter hastwo peaks at lower side and higher side of pore size, while the SiCfilter has a single sharp peak. This shows that the SiC filter has astructure having substantially the uniform pore size and the cordieritefilter has a structure having ununiform pore size distribution.

[0086] An inner structure of the cell wall in the SiC filter is shown inFIG. 12a, while an inner structure of the cell wall in the conventionalcordierite filter is shown in FIG. 12b. As seen from FIG. 12a, pores hhaving substantially a constant pore size are connected to each other inthe cell wall of the SiC filter, which are easy to pass a fluid such asan exhaust gas or the like. In the cordierite filter, as seen from FIG.12b, pores h having various pore sizes are connected to each other inthe cell wall, while closed pores hc having a smaller pore size areindependently existent therein.

[0087] In case of the conventional checkered honeycomb cordieritefilter, the interconnected pores in the cell wall contribute to decreaseΔP3 in the aforementioned equation (1) at a state that the particulatesare not yet deposited on the cell wall, which makes the initial pressureloss ΔPi small at any flow rates Va. However, as the exhaust gasgradually passes through the filter, the particulates included in theexhaust gas are concentrically deposited on surface portions of the cellwall existing pores having a large pore size located near to the surfaceof the cell wall to cover such surface portions, and hence the pass ofthe exhaust gas through portions of such large-size pores covered withthe particulate is obstructed to rapidly increase the pressure lossthough the pores having relatively small pore size located near to thesurface of the cell wall contribute to decrease the pressure loss.

[0088] On the other hand, in the checkered SiC honeycomb filteraccording to the invention having the same cell structure as in thecordierite filter, the initial pressure loss ΔPi is equal to that of thecordierite filter. However, since the pore size of the interconnectedpores in the cell wall is substantially constant and the pore sizedistribution is uniform, when the exhaust gas is passed through thefilter, the particulates included in the exhaust gas are equallydeposited over the full surface of the cell wall, so that there iscaused no rapid increase of the pressure loss as in the cordieritefilter. And also, the value of the pressure loss becomes lower than thatof the cordierite filter.

[0089] In FIG. 13 is shown a relation between initial pressure loss ΔPi(mmAq) and air flow rate Va (m/sec) at 20° C. in the checkered SiChoneycomb filter having a side of 33 mm and a length of 150 mm and acell structure Cs (mil/cpi) of 17/100, 14/200 or 12/300. As seen fromFIG. 13, the initial pressure loss is proportional to the air flow ratesupplied to the filter. And also, the value of the initial pressure losslowers as the cell number becomes large and the thickness of the cellwall becomes thin. In other words, the pass of air through the filterbecomes more easy as the value of the initial pressure loss becomeslower. This is true in the results shown in FIG. 10. That is, the changeof pressure loss becomes small as the value of the initial pressure lossbecomes lower.

[0090] In FIG. 14 is shown an influence of a diameter φ of a filter inthe checkered SiC honeycomb filter upon the pressure loss ΔP when theparticulate included in the exhaust gas is caught by the filter having acell structure Cs (mil/cpi) of 14/200 with the lapse of time, wherein acurve P4 is a case that the filter diameter φ is 144 mm, and a curve P5is a case that the filter diameter φ is 165 mm, and a curve P6 is a casethat the filter diameter φ is 190 mm. As seen from FIG. 14, the pressureloss and the change thereof can be made small as the volume of thefilter becomes larger.

[0091] As previously mentioned, the total volume of the filter isdetermined by the engine swept volume of the internal combustion engineused. In the invention, it is favorable that the total volume of thefilter is set to ¼-2 times the engine swept volume of the internalcombustion engine when cell structure of the filter is the same. Whenthe total volume is less than ¼ times the swept engine volume, thepressure loss ΔP becomes too large as seen from FIG. 14 to considerablydegrade the fuel consumption, while when the total volume exceeds 2times the swept engine volume, the pressure loss can be made small, butthe total volume of the exhaust gas cleaning device inclusive of thefilter is too large and hence such a device can not be disposed in theexhaust emission path for the internal combustion engine.

[0092] In FIG. 15 is shown a relation between oxygen concentration (%)in an exhaust gas and a particulate burning temperature (° C.) forattaining a regeneration ratio of not less than 80% (shown by a shadowedregion) in the checkered honeycomb SiC filter after a given amount (10g/L) of the particulate is caught by the filter. In this case, the SiCfilter has a cell structure Cs (mil/cpi) of {fraction (14/200)} and aregeneration gas having various oxygen concentrations at a temperatureof 150° C. as a substitute for the exhaust gas is introduced into thefilter at a flow rate Va of 2.5 m/sec. Moreover, the term “regenerationratio” used herein means a percentage of filter weight after theintroduction of the regeneration gas to filter weight before theintroduction of the regeneration gas.

[0093] In FIG. 15, a shadowed region of a curve a is a region forattaining the regeneration ratio of not less than 80% when particulatesdeposited on the SiC filer from a diesel engine using a gas oilcontaining a fuel additive is burnt by the glow plug 34, while ashadowed region of a curve b is a region for attaining the regenerationratio of not less than 80% when particulates deposited on the SiC filerfrom a diesel engine using only a gas oil containing no fuel additive isburnt by a heater.

[0094] As seen from the curve b in FIG. 15, when using only the gas oilas in the conventional technique, in order to attain the regenerationratio of not less than 80%, the oxygen concentration in the exhaust gasis required to be not less than at least 15% and also it is required toheat the filter above about 650° C. by the heater for burning theparticulate. On the contrary, as seen from the curve a in FIG. 15, inorder to attain the regeneration ratio of not less than 80% according tothe invention, the gas oil containing the fuel additive is used in thediesel engine, so that the particulate burning temperature is sufficientto be at least 150° C., and also even when the oxygen concentration inthe exhaust gas is as low as 3%, it is enough to heat the filter at atemperature considerably lower than that of the curve b by the heater orthe glow plug.

[0095] According to the embodiment shown in FIG. 4, the usual size ofthe filter can be utilized as it is without increasing the volume of thefilter, so that the regeneration system for the exhaust gas cleaningdevice of the illustrated embodiment is effective to a vehicle having alarge displacement such as truck and bus having a displacement of 12liter. Although the illustrated embodiment is described with respect tothe branched structure of three regeneration units, the invention maytake a branched structure wherein two regeneration units are arrangedside by side in the exhaust emission path 11 as shown in FIG. 16a or astructure of arranging a single regeneration unit in the exhaustemission path 11 as shown in FIG. 16b.

[0096] The two branched structure shown in FIG. 16a is used in a truckhaving a displacement of, for example, 7 liter, wherein the control ofthe two regeneration units can be carried out by the same method as inthe three-branched structure shown in FIG. 4. That is, the flow controlvalve 26 and the glow plug 34 in each of the two regeneration units arecontrolled by a command from the control unit (not shown), whereby theexhaust gas is cleaned by the filter in one of the regeneration units ata non-worked state of a valve body 26 a in the flow control valve 26,while the filter of the other regeneration unit at a worked state of avalve body 26 a of the flow control valve 26 is regenerated by heatingsuch a filter through the glow plug 34.

[0097] The single structure shown in FIG. 16b is used in a passenger carhaving a displacement of, for example, 3 liter, wherein the control ofthe regeneration unit is carried out by switching on or off only theglow plug 34 without using the flow control valve because thepurification of the exhaust gas and the regeneration of the filter aresimultaneously carried out.

What is claimed is:
 1. An exhaust gas cleaning honeycomb filter for anexhaust gas cleaning device of a regeneration system, wherein theexhaust gas cleaning honeycomb filter is comprised of a porous siliconcarbide sintered body, and wherein the regeneration system is disposedin an exhaust emission path of an internal combustion engine that uses agas oil containing a fuel additive and that emits an exhaust gas intothe exhaust emission path, the exhaust gas honeycomb filter collectingparticulates included in the exhaust gas, and wherein the regenerationsystem includes a heating means for the filter.
 2. An exhaust gascleaning honeycomb filter according to claim 1, wherein the heatingmeans is selected from a heater and a glow plug.
 3. An exhaust gascleaning honeycomb filter according to claim 2, wherein the heater is aspiral-shaped resistance heating heater.
 4. An exhaust gas cleaninghoneycomb filter according to claim 1, wherein the exhaust gas cleaninghoneycomb filter has a total volume corresponding to ¼-2 times an engineswept volume of the internal combustion engine.
 5. An exhaust gascleaning honeycomb filter according to claim 1, wherein the heatingmeans is disposed in front of an upstream end face of the exhaust gascleaning honeycomb filter to heat the exhaust gas so as to obtain aparticulate burning temperature of at least 150° C. sufficient forattaining a filter regeneration ratio of not less than 80%.
 6. Anexhaust gas cleaning honeycomb filter according to claim 1, wherein theexhaust gas cleaning honeycomb filter has a cell structure such that acell number per square inch is not less than 100 cells and a thicknessof a cell wall is not more than 0.43 mm.
 7. A method of filtering anexhaust gas comprising: heating an exhaust gas cleaning honeycomb filterof a regeneration system with a heating means, the regeneration systemlocated in an exhaust emission path of an internal combustion engine;feeding an exhaust gas from the internal combustion engine, which uses agas oil containing a fuel additive, into the exhaust emission path andthrough the exhaust gas cleaning honeycomb filter of the regenerationsystem; and collecting particulates included in the exhaust gas with theexhaust gas cleaning honeycomb filter, wherein the exhaust gas cleaninghoneycomb filter is comprised of a porous silicon carbide sintered body.8. The method of filtering an exhaust gas according to claim 7, whereinthe heating means is selected from a heater and a glow plug.
 9. Themethod of filtering an exhaust gas according to claim 8, wherein theheater is a spiral-shaped resistance heating heater.
 10. The method offiltering an exhaust gas according to claim 7, wherein the exhaust gascleaning honeycomb filter has a total volume corresponding to ¼-2 timesan engine swept volume of the internal combustion engine.
 11. The methodof filtering an exhaust gas according to claim 7, wherein the heatingmeans is disposed in front of an upstream end face of the exhaust gascleaning honeycomb filter to heat the exhaust gas so as to obtain aparticulate burning temperature of at least 150° C. sufficient forattaining a filter regeneration ratio of not less than 80%.
 12. Themethod of filtering an exhaust gas according to claim 7, wherein theexhaust gas cleaning honeycomb filter has a cell structure such that acell number per square inch is not less than 100 cells and a thicknessof a cell wall is not more than 0.43 mm.